Waste stacks are generated by many types of industrial processes, often as a result of the extraction of valuable materials. The waste stacks are frequently piles of economically invaluable material left over from the industrial processes. For instance, power plants often generate waste stacks of ash. The ash is left over when energy is extracted from fuel by burning. Mining processes also often generate waste stacks. The waste stacks contain minerals left over after a valuable metal or mineral is extracted from the mined earth materials. For example, phosphorus mines often result in waste stacks containing predominantly gypsum. The waste stack gypsum is a relatively invaluable mineral left over after phosphorus is removed from the mined materials.
In many instances, waste stacks are formed as follows. First, the residual or waste material is combined with water to form a waste slurry. This waste slurry is then flowed to a settling pond where the solids contained in the waste slurry settle out. Water also evaporates from the settling pond. Over time, the settled solids leave behind a stack of waste material. Some water is retained in the settled waste material which makes up the waste stack. This process of deposition settling and evaporation is repeated until the resulting waste stack is too large for the process to economically continue, or is terminated for other reasons. If needed, a new waste stack is started and grows in a similar fashion. FIG. 1 shows a mine 12 which has been in operation for a significant period of time and is surrounded by a number of waste stacks 14. The individual waste stacks 14 are often huge, sometimes comprising millions of cubic yards. The amount of material currently stored in waste stacks is enormous, and it continues to increase as mining and other industries continue to produce and develop new operations.
A problem associated with waste stacks is toxic metal migration. The actual percentage of water-soluble toxic metals in a given waste stack is usually very small; for example, less than 0.1 percent. However, because the waste stacks are often very large, the total amount of toxic materials in a waste stack is often large enough to present some risk to surrounding areas and ground water. These risks arise in part due to potential metals migration from the transport of liquid slurry to the waste stack. The slurry water may percolate into the soil in addition to evaporating or remaining in the waste stack.
Toxic metals potentially found in waste stacks include but are not limited to Pb, Hg, Cd, Fe, As, Se, Cu, Cr, Ni, Zn, Co, Mn and Ag. Over time such metals can leach out of the waste stacks and into ground water. Thus, it is desirable to keep the metals within or near the waste stacks to minimize the danger posed by such metals.
Keeping the metals within or near the stack is often difficult, especially since the metals may be present in water-soluble forms. Such water-soluble forms can migrate as metal solutes whenever water moves through the stacks. Since the stacks are frequently exposed to water, either in the form of rain or in the form of waste water deposited on the stacks, water-soluble metals or metal compounds present in the stacks are exposed to conditions which may encourage their migration. In some situations, metals have already begun migrating out of existing waste stacks and into a boundary zone or layer below the waste stacks. Thus, it is desirable to have a method which will not only inhibit further migration of metals from the waste stacks, but which will also inhibit the migration of metals that are in a boundary layer beneath the waste stacks.
One prior art method for containing metals within a waste stack is to insert a liner beneath the waste stack. However, since the waste stacks need to be lifted or removed to utilize this method, the method is too expensive and time-consuming to be a practical or economical treatment for many waste stacks.
A principal object of this invention is to provide a treatment method for containing metals within an existing waste stack or waste stack affected zone. Meeting this objective is complicated by the requirement that any acceptable treatment method should not result in emissions of toxic materials or malodorous gases. The treatment method should also be economical and preferably not result in substantial amounts of additional waste. Additionally, the difficulty in finding an acceptable treatment method is exacerbated by other chemical constituents of the waste stack which may counteract various attempts to chemically treat the waste stack. Thus, there has long been a need in the art of industrial waste management for an acceptable method for dealing with large waste stacks containing metal contaminants in low concentration levels; and in particular, a need in the art of phosphate fertilizer production for dealing with large gypsum waste stacks containing small concentrations of metals.